The present invention relates to electrical transformers in general, and in particular to an electronic transformer which may be conveniently controlled to exhibit selected properties.
In basic principle, the conventional transformer consists of two coils wound on the same iron core. An alternating current in one winding sets up an alternating magnetic flux in the core, most of which is linked with the other winding and induces in it an alternating emf. Power is thus transferred from one winding to the other via the flux in the core. The winding to which power is supplied is called the primary, that from which power is delivered is called the secondary, and either winding may be used as the primary.
An ideal conventional transformer has no resistances in the windings, and therefore no winding power losses, no core losses such as hysteresis and eddy-current losses, and the coefficient of coupling between the windings is equal to unity. In addition, the permeability of the core is sufficiently high that, theoretically, no emf is necessary to produce the required mutual flux, and the windings have no capacitive effect. Thus, in an ideal transformer the power input to the primary winding equals the power delivered by the secondary winding, or the voltage across one of the windings multiplied by the current therethrough equals the voltage across the other winding multiplied by the current therethrough. As is well known, however, conventional magnetic transformers are far from ideal.
In particular, such transformers exhibit distributed winding capacitances, leakage inductances, core losses and winding capacitance, so that the power delivered by the transformer is less than the power input thereto, and the voltage and current at the output from the transformer are not truly representative of those at the input thereto. For example, in any actual magnetic transformer the flux lines are not confined entirely to the core, but instead some return through the air to their associated windings. Thus, all of the flux generated by the primary winding is not coupled with the secondary winding, so that power losses are incurred. In addition, and irrespective of flux coupling losses, the power output of a conventional transformer is necessarily less than the power input because of unavoidable losses in the form of heat, which consist of resistance heating in the primary and secondary windings and hysteresis and eddy current heating in the core. Hysteresis may be minimized by the use of an iron core having a narrow hysteresis loop, and eddy current by laminating the core, but in spite of these efforts significant losses nevertheless occur.
Because of such disadvantages, it is difficult to design a magnetic transformer to provide, for example, accurately controlled impedance matching and power transmission characteristics, since all of the winding capacitances, winding resistances, leakage inductances and core losses affect the output obtained from a transformer in response to a particular input thereto. Consequently, in the design of such transformers only a best approximation response may be sought and obtained.